1. Field of the Invention (Technical Field)
The present invention relates to passive optical communications, in particular to full-duplex communications on a single electro-magnetic beam and to the use of circular polarization keying as a modulation format.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Issued patents that have addressed the need for full-duplex communications on a single electro-magnetic beam include U.S. Pat. No. 4,882,770 to Miyahira, et al. entitled, xe2x80x9cWireless Optical Communication System Utilizing a Single Optical Carrier Frequency,xe2x80x9d which discloses multiplexing for a full-duplex wireless optical short-range communications system by using two oscillators, one to modulate the audio data on a subcarrier frequency, and the other to modulate the subcarrier frequency on an optical infrared carrier frequency. Time division multiplexing is used as the modulation format. U.S. Pat. No. 5,121,243 also to Miyahira, et al. entitled, xe2x80x9cWireless Optical Communication System,xe2x80x9d also discloses a wireless optical short-range communications system using time division multiplexing. U.S. Pat. No. 5,146,358 to Brooks entitled, xe2x80x9cOptical Communications System and Method for Transmitting Information Through a Single Optical Wave Guide,xe2x80x9d discloses simultaneous bi-directional, multi-channel communication. However, the data is either amplitude or frequency modulated. U.S. Pat. No. 5,272,555 to Suzuki entitled, xe2x80x9cBi-Directional Optional Transmission Method and Apparatus Therefore,xe2x80x9d discloses bi-directional communication through a single optical path using either frequency modulation, amplitude modulation, or pulse modulation. U.S. Pat. No. 3,920,983 to Schlafer, et al. entitled, xe2x80x9cMultichannel Optical Communications System Utilizing Multi-Wavelength Die Laser,xe2x80x9d discloses a communications system having frequency multiplexed modulated signals in a single beam.
U.S. Pat. No. 5,966,224 to Hughes, et al. entitled, xe2x80x9cSecure Communications With Low-Orbit Spacecraft Using Quantum Cryptography,xe2x80x9d discloses using polarization as well as delay to accomplish secure communications. Vertical polarization and right-hand circular polarization is disclosed to distinguish between zeros and ones, or in the alternative, left-hand circular polarization and horizontal polarization to represent zeros and ones. U.S. Pat. No. 5,850,441 to Townsend, et al., entitled, xe2x80x9cSystem and Method for Key Distribution Using Quantum Cryptography,xe2x80x9d discloses a cryptograph communication system using polarization modulation. U.S. Pat. No. 5,978,121 to Fischer, et al., entitled, xe2x80x9cMethod and Device for the Optical Transmission of Data Over Distances in Space,xe2x80x9d discloses the use of phase modulation to accomplish optical transmission. The light signal containing the information is circularly polarized, but does not use circular polarization as the modulation scheme. U.S. Pat. No. 4,689,625 to Barmat entitled, xe2x80x9cSatellite Communications System and Method Therefore,xe2x80x9d discloses transmitted and received signals operating in the same frequency band but with opposite linear or opposite handed circular polarizations, therefore requiring low power. U.S. Pat. No. 4,888,816 to Fica, Jr., entitled, xe2x80x9cTwo-Way Optical Communication System for Atmospheric Use,xe2x80x9d discloses a two-way communication link with reduced light linkage and uses circular polarization, but with an amplitude modulation scheme.
None of these patents implement the unique differential circular polarization keying format and novel architecture for full-duplex communications on a single electro-magnetic beam as in the present invention. Additionally, conventional modulation formats for open air optical communications use amplitude shift keying (ASK), on-off keying (OOK), phase shift keying (PSK), or frequency shift keying (FSK). These modulation formats either only register signal for one state or they require a complex coherent detection system, the advantage of which falls away rapidly if there are any wave front distortions in the atmosphere. In contrast, circular polarization keying (CPK) uses simple paths of optical elements to separate the two circular polarization states and can direct each polarization state to a separate detector. Thus, the signal-to-noise ratio is increased by between 3 and 6 decibels simply by subtracting the output from those two channels. Each channel can be operated in a simple direct detection mode, thus avoiding the complexities of a coherent detection system. Passing circularly polarized light through a quarter-wave plate converts circularly polarized photons into linearly polarized photons. Furthermore, each left-hand circularly polarized photon is converted into a linearly polarized photon with the same polarization direction, while each right-hand circularly polarized photon is converted into a linearly polarized photons whose direction is orthogonal to the direction of the converted left-hand circularly polarized photons. Thus, CPK eliminates the difficulties associated with aligning linear polarization sensitive elements. This is particularly advantageous when certain polarization sensitive elements are used, such as Faraday Anomalous Dispersion Optical Filters, that provide ultra-high background rejection.
The present invention is a system allowing for full-duplex communications on a single electromagnetic beam in a communications system. This is accomplished by choosing data formats such that the forward data format (or xe2x80x9cuplinkxe2x80x9d in the embodiment presented herein) is invisible to the return data format (or xe2x80x9cdownlinkxe2x80x9d in the embodiment presented herein). The present invention offers the potential of doubling the data rate of these systems while keeping the power consumption at the remote end very low. The present invention is useful for a passive retro-modulated communications system where a carrier beam is transmitted to a reflector that modulates the infinite carrier and returns it back to the source. In such a system full-duplex communications is often desired, while in conventional systems, time multiplexing is required between the transmit and receive phases. Time multiplexing reduces the data rate in each direction. The present invention overcomes this drawback. No time multiplexing is necessary because the data format in the forward direction is invisible to the data format in the return direction.
The present invention further implements a unique binary modulation format for optical communications. Using CPK modulation the zeros and ones are each represented by one of the two orthogonal circular polarization states. For example, a zero can be represented by right-hand circularly polarized light, and a one represented by left-hand circularly polarized light, or vice-versa. Thus, the presence of a zero or one can be determined by measuring the circular polarization of the incident light.
The present invention is a method of full-duplex electromagnetic communication wherein a pair of data modulation formats are selected for the forward and return data links respectively such that the forward data electromagnetic beam also serves as a carrier for the return data. The present invention is further a method of encoding optical information wherein right-hand and left-hand circular polarizations are used to represent binary states.
One embodiment which implements the present invention is a full-duplex optical communication system wherein a laser diode transmitter and downlink receiver are located on the ground and a retro-modulator and uplink receiver are located on an object in space. The transmitter transmits a linearly polarized electro-magnetic beam to the object in space and a first quarter-wave plate converts the linearly polarized beam into a circularly polarized beam. A telescope directs the electro-magnetic beam to the object as well as collects the beam that is returned from the object. The retro-modulator at the object directs the incident beam back to the telescope and also assigns left-handed and right-handed circular polarizations to the returned beam according to binary data bit information. An aperture sharing element is located between the first quarter-wave plate and telescope to separate the transmitted and returned electro-magnetic beams. A second quarter-wave plate converts the returned left and right-handed circularly polarized beams into two orthogonally polarized beams respectively. A first beam splitter is located prior to the second quarter-wave plate and sends a small percentage, such as less than 1%, of the returned beam to a first Faraday Anomalous Dispersion Optical Filter and the remaining percentage of the returned beam to the second quarter-wave plate. A second polarizing beam splitter separates the two orthogonally polarized beams that are received from the second quarter-wave plate into a second and third Faraday Anomalous Dispersion Optical Filter respectively. The two received orthogonally polarized beams from the second and third Faraday Anomalous Dispersion Optical Filters are then subtracted from one another.
The transmitter is preferably a semi-conductor diode laser and the retro-modulator is preferably a liquid crystal retro-modulator. The aperture sharing element preferably comprises a mirror with an opening defined therein. Full-duplex communication is achieved on a single electro-magnetic beam due to the forward and return links being formatted so that they are invisible to one another.
A primary object of the present invention is full-duplex communications on a single electro-magnetic beam.
Another primary object of the present invention is to encode optical information by assigning right-hand and left-hand circular polarizations to represent binary states.
A primary advantage of the present invention is that the forward and return data formats are invisible to one another enabling full-duplex communications on single electromagnetic beam.
Another primary advantage of the present invention is that the use of a circular polarization keying modulation format increases the signal-to-noise ratio by approximately 3 to 6 decibels and allows for direct detection thereby avoiding the complexities of a coherent detection system, and further eliminates the difficulties associated with aligning linear polarization sensitive elements.
Another advantage of the present invention is that through the use of Faraday Anomalous Dispersion Optical Filters, ultrahigh background rejection is accomplished.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.